Alloy base for oxide-coated cathodes



Patented Dec. 18, 1928.-

4 UNITED STATES PATENT. OFFICE.

GEORGE W. HALLOCK, OF BLOOMIIELD, JERSEY, ASSIGNOR J30 WESTINGHOUSE LAMPCOMPANY, A CORPORATION OF PENNSYLVANIA.

ALLOY BASE FOB OXIDE-COATED CATHODES.

No Drawing.

This invention relates to electron-emittingcathodes of the oxide coatedtype, such as are used in thermionic discharge devices, and moreparticularly to an electrically conduc- 5 tive core or base for suchcathodes.

Heretofore in the manufacture of oxide coated cathodes'ithas been theusual practiceto employ a core of nickel or platinum-iridium alloy.Nickel has very largely supersededthe use of platinum-iridium, due tothe excessive expense of the platinum. Nickel has the d1S advantage,however, in that its specific resistance is such that the voltage-amperecharacteristics required for the large size filaments cannot be readilyobtained without reducing the thickness of the core to such an ex- 'tentthat it is mechanically weak or, if proportioned for proper tensilestrength, causing it to o crate at an excessively high tem-. 2 perature,elther of which result in short life. For instance, with large sizes ofribbon, in order to obtain the required voltage and amperagecharacteristics, and the requisite surface area to maintain the properoperating temperature, it is necessary to roll the core into'a ribbon ofconsiderable width and of an extreme thinness, below 1 mil. Greatdifficulty is involved in uniformly rolling the material to this degreeof thinness and the edges become frayed and feathered so that the ribbonis easily torn and does not have 7 sufficient strength to withstand thestrain required to maintain it taut on the mount. The ribbon is,therefore, made thicker and narrower than theoretically required inorder to increase the tensile strength but due to the decrease in} heatradiating surface thus occasioned, the cathode operates at anexcessively high temperature. This high filament temperature results inbombardment of the cathode by back emission from the grid or plate,caused by the thermionically active coating on the cathode being thrownofi at the high operating temperature thereof and depositing on thegrid, and the heating of the grid, to an electron emitting temperatureby heat radiated .from the overly heated. This back bombardment rapidly:

cathode. destroys the filament.

Difliculty hasalso been experienced in the smaller size filaments whenemploying nickel cores, in causing the oxide coating to adhere, since inorder to obtain the required voltageamperage characteristics, it isnecessar to v go to the other extreme, and make the ribbon Applicationfiled October 27, 1927. Serial No. 229,276.

narrow and thick, rather than broad and thin. The oxlde coating does notadhere as readily 'to the narrow ribbon as it does to the broadersurface.

The hot strength of nickel is low, moreover,

which contributes materially to the failures due to breakage of both thesmaller sized and the extremely thin nickel ribbons.

One of the objects of the present invention 1s to overcome the abovementioned diiiiculties and to produce a metal core for an oxide coat-'ed cathode which will have the proper specific resistance to enable therequired voltage-amperage characteristics to be readily obtained.

at the proper operating temperature and whlch will be strong and rugged.

A further object is to provide an electrical- I ly conductive core whichwill have a high tensile strength while hot.

A stlll further object is to provide a thermionlcally active cathodewhich will have a large electron-emitting surface area for av ploy analloy having a higher specific resistance than pure nckel, whereas, forthe smaller slze'filaments, Iemploy an alloy having a lower specificresistance than nickel.

Thus, for instance, in producing cores for cathodes of large size,'thatis, having a filament weight corresponding to an 8 mil wire, 1n order tomeet the voltage and amperage re-' qulrements which have becomestandardized in the radio industry and maintain proper operat-mgtemperature of the filament, 'it would be necessary to flatten an 8 milnickel wire to a width of about 40 mil and a thickness of less than 1mil. This is not practical, however, due to the low tensile strength ofnickel, and the extreme fragility of a ribbon of this thinness. I havefound, however, that certain alloys, such as an alloy of silicon andnickel or manganese and nickel in the proper proportions have a higherspecific resistance than pure nickel and, therefore, that it is possibleto employ a thicker ribbon of substantially the same width and whichwill, therefore, have increased tensile strength in proportion to theincrease in size. Siliconnickel and manganese-nickel have greater hotstrength than ure nickel moreover so that the strength of tlie filamentis still further increased.

For the. large'size wires, silicon-nickel is preferred since its changein specific resistance for various percentages is very slight. An alloycontaining about 3% of silicon I have found very satisfactory, but thepercentage may run from 2 to 6% without materially a tering theelectrical conductivity of the metal. This isadvantageous since iteliminates the necessit of maintaining exact proportions in thefabrication of the wire. Manganese nickel with a manganese content of 4%is also satisfactory for large filaments,

but the proper percentage of manganese must be more carefullymaintained, since the con- .ductivity varies considerably with thechange in manganese content. I

Pure nickel ribbon of a weight corresponding'to a 6 or 8' mil Wire has aspecific resist-. ance of about 5.35 ohms per sq. cm., whereas, thealloys which I prefer for filaments of this: weight have a specificresistance of about 6.

For small size wires, it is desirable that the specific resistance of'the metal core be lower 'meet the standard requirements.

than that afiorded by pure nickel in order to of pure nickel, as the Weiht of the filament is decreased, for use 1n tubes of smaller output, thethickness of the filamentary core increases and the .width decreases sothat for the smallest size filaments the thickness approaches' thewidth. When these conditions are obtained,-diificulty is experienced incausing the oxide coating to properly adhere to the small flat surfacepresented by the narrow ribbon. In such case, I have found tlrat analloy such as an alloy of manganese and nickel containing about 2% ofmanganese,

which has a lower specific resistance than nickel, may be shapedinto arelatively broad and thin ribbon while maintaining the required voltageand amperage conditions and the proper operating temperature. This broadribbon presents a more extensive sur-' face to which the oxide coatingwill adhere more tenaciously. It also increases the electron emittingarea of the oathode and thereby enhances the electron emit-.

ting property of the coated cathode. In these small size wires theincreased tensile strength afforded by the alloy core increasesvery-materially the life of the device in which it is employed.

I have further found that the silicon or jmanganese-nickel alloy coresare not as susceptible to variations 1n their specific resistances byinternal changes in the core as is nickel. 4 In thecase of pure nickel,the conductivity,variesaccording' to the particular In the caseheattreatment and mechanical working to which it is subjected; It is thepractice in the manufacture of electron discharge devices employingoxide sary to take into consideration the change in conductivityeffected by this high temperature treatment. Since this treatment is notuniform in all tubes, the electrical conductivity of filament .cores ofthe same size is notuniform and this non-uniformity has causedconsiderable difliculty.

In the case of silicon-nickel or manganese nickel alloy, I have foundthat this high temperature treatment does not materially vary theelectrical conductivity and much greater uniformity in the electricalcharacteristics of the cathodes is obtained and the design of the I coreconsiderabl simplified.

One of the difficulties, which has'been mentioned before, when em loying large oxide coated cathodes with n ckel cores has been the hightemperature at which the cathodes necessarily operate by virtue of thefact that they must be made thicker and narrower than they should betheoretically, in order to impart the necessary mechanical strengththereto. The proper o erating temperature, particularly in tubes avin alarge power output, must be carefully 0 served, since if the v Itemperature is too high the oxide coating sputters or otherwise becomestransferred over to the other electrodes. These other electrodes becomehighly heated by radiation from the cathode so that they themselvesbecome thermionically active and the electrons emitted therefrom bombardthe filament and rapidly disintegrate the same so that it quick- 1y urnsout. Due to the greater specific resistance of silicon-nickel ormanganese-nickel when employed as a core for filament of large size, theheat radiating area is increased for the same voltage and currentconditions and consequently the filament operates at a lowertemperature, whereby the difficulties due to back emission areeliminated.

It should be understood that while siliconand manganese have'beenspecified as the particular alloying ingredients in definiteproportions, that the invention is not limited to these specificdetails, butequivalent metals may be employed andother proportions, de-

5 thermionically active cathode comprising an alloy of nickel andsilicon.

2. An electron-emitting cathode comprising an alloy of nickel andsilicon and a coating thereon of thermionically active ma- 10 terial.

3. A metal core for an electron-emitting cathode of the oxide coatedtype composed of an alloy of nickel and about 3% of silicon.

4. An electron-emitting cathode comprising a core composed of an alloyof a metal of the group containing nickel and up to 6% of silicon.

In testimony whereof, I have hereunto sub scri lzied my name this 26thday of October, 192

GEORGE W. HALLOCK.

